If a pendulum's swinging quite free
Then it's always a marvel to me
That each tick plus each tock
Of the grandfather clock
Is 2 pi root L over g.

And some Intarweb-nerdy ones, like:

There once was a man from Nantucket
whose walrus oh so luved its bukket.
One day walrus groaned,
it had just gotten pwned;
Said the lolcat "O HAI, I JUST TUK IT".

There are, of course, lots of dirty ones.  Of those, I only really appreciate those which a) keep the language fairly clean, and b) are clever, e.g.:

There was a young girl from Rabat,
who had triplets, Nat, Pat and Tat;
It was fun in the breeding,
But hell in the feeding,
When she found she had no tit for Tat.

Hope you LOLed.